The present invention relates to fluidized bed combustors in general and, more particularly, to a fluidized bed combustion apparatus specifically adapted for intermittent operation and rapid start-up, and to a method for operating same in a residential heating system.
The use of coal for residential heating is increasing as the availability of oil and natural gas decreases and the price of oil and nature gas increases. Coal stoves, whether batch fed or stoker fed, are not very efficient. The relatively poor controlability of air to coal stoichiometry typical of coal stoves results in high carbon loss and high stack heat loss. Further, the need to maintain a smoldering fuel bed during periods of no heating demand, such as overnight, in order to ensure rapid response upon the call for heat, results in a wasteful consumption of fuel.
Fluidized bed combustion is well known in the art as providing a very efficient and simple method of burning coal. In a typical fluidized bed combustor, a particulate fossil fuel, such as coal, is introduced into a bed of particulate material disposed in a chamber above a perforated or slotted bed support plate. Fluidizing air, which also serves as combustion air, is introduced into the bed upwardly through the bed support plate. The velocity of the fluidizing air is maintained above the minimum fluidization velocity of the bed material, that is, above that velocity at which the entire bed of particulate material is suspended or floated by the fluidizing air above the bed support plate. Typically, heat exchange tubes are immersed within the bed to effect heat transfer from the combustion process to a heat exchange medium, most commonly water, flowing through the tubes. In some instances, heat exchange tubes may also be disposed in the freeboard region directly above the fluidized bed.
Fluidized bed combustors are increasingly being utilized as furnaces for steam generators to produce superheated steam for process heating and electric power generation. In such a furnace, crushed or pulverized coal is typically mixed with a sulfur sorbent for absorbing sulfur oxides formed during the combustion process. The particulate material making up the bed consists of unburned coal, fresh and spent sulfur sorbent, and residual ash from the already burned coal. The fluidizing air serves as combustion air. Water circulating through tubes forming the furnace walls and heat exchange tubes immersed therein is partially evaporated therein to form a steam and water mixture. The steam is then separated from the water and again passed in heat exchange relationship with the combustion gases produced in the bed for superheating.
Unfortunately, prior art fluidized bed furnaces, such as those utilized for steam generators, are not suitable for applications to residential or small industrial heating applications. For either space heating or small industrial process heating applications, rapid response to heating load demand and rapid cycling capability is critical. The furnace must be adapted for intermittent operation with rapid start-up capability after relatively short or extended periods of shut-down. The prior art fluidized bed furnaces are generally very sluggish in response and incapable of rapid start-up as the particulate bed must typically be preheated to ignition temperature prior to restarting the bed after shutdown, or the bed maintained in a smoldering state as in a coal stove.